1. Field of the Invention
The invention is related to microelectromechanical systems (MEMS) and more particularly to manufacturing MEMS structures.
2. Description of the Related Art
In general, co-fabrication of microelectromechanical systems (MEMS) with integrated circuits achieves higher performance systems at lower cost. The use of materials for structural and sacrificial layers (e.g., poly-silicon-germanium and poly-germanium) that have a sufficiently low thermal budget (e.g., can be processed using steps at temperatures below 450 degrees Celsius) will not affect the integrated circuits and allows the fabrication of MEMS by surface micromachining after the completion of an integrated circuit process, such as complementary metal oxide semiconductor (CMOS). Encapsulation of the MEMS is feasible using any of a variety of technologies (e.g., die-level packaging or wafer-level packaging using bulk wafer caps, micro-assembled caps, in situ caps, or other suitable techniques). The resulting device is encapsulated by a microcap with a cavity containing the MEMS.
For proper MEMS device operation, the cavity pressure should remain stable during a specified lifetime of the device. Typically, a relatively high temperature anneal is performed prior to encapsulation, to facilitate outgassing of any species adsorbed on the MEMS or integrated circuit surfaces or any mobile species located in the bulk of the surface films. The stability of cavity pressure is increased by introducing one or more getter materials in the cavity to scavenge residual gases during and after sealing of the cavity. A typical getter is a coating applied to a surface within the evacuated chamber. When gas molecules strike the getter material, they combine chemically or by adsorption. However, getter materials may not effectively scavenge all gases. For example, getter materials may be ineffective at scavenging gases that originate in the materials used in the integrated circuit metal stack, e.g., low-k dielectric layers. Certain getter materials may also be impractical to integrate as part of the manufacturing process. Typical passivation layers on the integrated circuit (e.g., low-temperature silicon nitride and silicon dioxide (SiO2) layers) are designed for mechanical protection and are not effective barriers to out-gassing during encapsulation or device operation. Accordingly, improved techniques for co-fabricating MEMS devices and integrated circuits are desired.